A Novel Load Sharing Control Technique for Paralleled Inverters Jingtao Tan, Hua Lin, Jun Zhang and Jianping Ying

Delta Power Electronics Center 238 Minxia Road, Caolu Industry Zone, Pudong, Shanghai, 201209, China

Tel: 86-21-58635678, Fax: 86-21-58630003, Email tan,jingtao@delta,com.cn

Abstract -A new control technique have been developed, which allows paralleled inverters to share linear or nonlinear load in a distributed ac power supply system. All paralleled inverters have the Same power reference, and are forced the output power to follow the same reference by using a PI controller in each inverter unit, so that all paralleled inverters can share the Same output power. The approach has significant advantages over existing methods, including the ability to improve reliability and reduce interconnection bus. Principle and scheme of the load-sharing technique is analyzed. The experimental resdlts of B three-unit prototype system show that the proposed scheme is correct.

I. INTRODUCTION

Continuous power supply systems have become increasingly important, especially for applications with sensitive and critical loads. A common practice to obtain a continuous power supply is to use a centralized unintempted power supply (UPS) system. This, however, is inflexible and can he unreliable for distributed loads. As the system load grows, the UPS needs to be replaced with a higher capacity one. Also, if the U P S fails, the entire system is affected.

The reliability as well as the power capability of the supply system can he increased by replacing a single UPS unit with multiple, smaller UPS units in parallel, resulting in a so-called distributed power system (DPS). A DPS has many desirable features such as expandability, modularity, maintainability, redundancy, and increased reliability. The technically challenging aspect of the DPS, however, is the load sharing among the parallel-connected inverters. Without a proper control scheme, each unit cannot share the load properly. Also, the load sharing is affected by non- uniformity of the units, component tolerances and variations in the connecting line impedances.

Although many methods of operating inverters in parallel can be found in the literature, there is yet no satisfactory method to achieve a truly distributed power supply system ['"I. We think a good DPS should meet the demands as follows:

(1) Circulation current between paralleled inverters is almost zero;

(2) Dynamic and static performance of the DPS is no lower than the single UPS, such as low THD (total harmonic distortion) and precision of output voltage;

(3) High reliable parallel configuration. As will be discussed in details by following sections, a

novel load sharing control technique for parallel inverters is put forward.

11. ANALYSIS OF POWR FLOW CONTROL PRINCIPLE

A. ConvenlionalPower Flow Confrol Theory

A model of two inverters parallel connection is shown in Fig.1, in which the inverter connects with the load via a filtering inductance. As well know, the complex power at the terminal vo in Fig.1 due to the ith ( i=l,or2) inverter is given by[''

Equations (I) and (2) indicate that if power angle is

small, the real power flow 4 i:; mostly dependent on the

power angle p i . On the contrary, the reactive power flow

Q, is mainly influenced by the amplitude of the inverter

voltage V, . This is a general conclusion on power flow

control in the power supplies system.

I I

Fig.1 Principle Diagram of Inverter Parallel Connection

Therefore, according the above conclusion, in order to control the power flow of inverter units in the parallel system, we should take meawnss to control the amplitude and phase angle of the output voltage of all inverter units effectively. Fig2 and Fig.3 are two kinds of system, which can meet these demands. Fig2 is an open loop control structure while Fig.3 is a clo:ied loop control structure. Obviously, whether in open or in close control structure, to change the amplitude VmJ and phase angle @, of the

0-7803-7754-0/03/$17.00 82003 IEEE 1432

voltage reference, can control the amplitude V, and phase

angle pi of the output voltage of the ith inverter. INVETER INVETER

1 2

I

Fig.2 Parallel system of inverter units using open Iwp contml "QY

Fig3 Parallel syStem of inverter units with the output voltage as feedback

Based on the above analysis, a general method on power flow control of inverter parallel system can be summarized as follows:

(1): To change the phase angle 'ZJi of the voltage

reference, can control the real power flow e from the ifh inverter.

(2): To change the amplitude V,, of the voltage

reference, can control the reactive power flow Q, from the

ith inverter.

B. Modified Power Flow Control Theory

High performance inverters have some important features, such as high voltage regulation precision, and low total harmonic distortion with various loads. Thus, the inverters are typically operated under feedback control to realize the desired output waveform.

However, it should he noted that the load voltage is usually used as feedback to construct closed loop control in the inverter. Thus, a parallel system of inverter units is shown in Fig.4. It is distinct from the parallel system shown in Fig.3, in which the output voltage of the inverter is used as the feedback.

.--.*,iJ Fig.4 Parallel SyStem of invencr units with the load voltage

feedback

Fig.5 Parallel system of inverter uni$ with current control Imp

Now, the problem is how to control the power flow Of inverter unit in Fig.4. Although, no matter what control strategy is adopted in Fig.3, the conclusion about the relationship between voltage reference and power flow is unchangeable, different conclusions will get when different control strategies is used in Fig.4.

Load voltage and inductor current dual-bops control is a typical control strategy for inverters. Parallel system of inverter units based on the control strategy is shown in FigS. Power flow control principle on the parallel system is discussed as follows.

In Fig.5, the IVR is the instantaneous voltage regulator, which enforces the load voltage to follow the voltage reference tightly. The output of IVR is supplied as the inductor current reference of the CR. The CR is the current regulator, which enforces the inductor current to follow the inductor current reference tightly.

Generally, the load voltage v, and the voltage

references of the ith inverter vmln can he expressed as:

v, = V,, sin wt v , ~ ~ = V,eln sin( cot + 'ZJ ")

Where: w is the angle frequency of the load voltage, 'ZJn is the phase difference between the voltage reference

of the nth inverter unit and the output voltage, v, is the

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amplitude of the load voltage and v,, is the amplitude of the iih voltage reference.

In additional, to simplify the analysis, some assumptions are given:

( I ) The N R is a proportional controller; the proportional gain is K, .

(2) "he CR has very perfect performance, so that the inductor current can follow the current reference completely.

Thus, the mathematical model of the nth inverter unit in Fia.5 can he exmessed as: I

i,,+ = K , *[V,, *sin(ui+@.)-V, *sinoil (3)

ILn = 54" (4) From the equation (3) and (4). we have:

-- ai," -K,sin(wf+@,) a VM"

Obviously, when the amplitude increment of the voltage reference is AV,+, the increment of the inductor current

A& can he expressed as:

Ai,," = LY,AV,~/, sin( 0 1 + @ " ) (7)

Usually, Q n is very little, so the Airefi in equation (7)

predominately produce active current in the load voltage

Similarly, when the phase angle increment of the voltage reference is A@", the increment of the inductor

current A i , can he expressed as:

vo .

Ai,@ = K,Vamp,AQ cos( 01 + Q n ) (8)

So the Ai,+ in equation (8) predominately produce

reactive current in the load voltage vo . Based on the above analysis, in Fig.5, a general method

on power flow control of inverter parallel systcm as follows: (1) To change 'the phase angle Qi of the voltage

reference can control the reactive power flow Qi from the i fh inverter.

(2) To change the amplitude V,, of the voltage

reference can control the active power flow 6 from the iih inverter.

111. LOAD SHARING SCHEME

A. Disiribuied Logic Confguraiion

Parallel system of inverter units has different kinds of parallel configuration, such is the central control configuration, the maser-slave control configuration, the distributed logic control contiguntion and independent control configuration. Considering the cost, reliability and complexity of implementation,, the distributed logic configuration shown in Fig.6 is adopted in the paper. It has only two interconnection buses to transfer the related information among all the paralleled inverter units. The phase bus is used to transfer the phase angle information, so that the voltage references of all the paralleled inverter units have the same frequency and phase angle. The power bus is used to transfer the active power reference information, so that the voltage references of all the paralleled inverter units have the same active power reference.

I n+ 1 NVETE

I Load Fig.6 Parallel syslem of inverter based on dirhibuted logic

configuration

B. Load Sharing Conhol Scheme

Control block diagram for realizing the proposed load sharing control scheme is shown in Fig. 7. It is augmented on the dual-loops inductor current control scheme shown in Fig. 5.

Rm is the real-means-square (RMS) voltage regulator, which ensures the RMS of the load voltage to have high precision. The output of RVR is regard as the active power reference P, , which is transferred to the power bus to get

the average value of all the active power reference in the paralleled system.

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(9)

PR is the active power regulator. is supplied as

the input of the active power regulator. The output of PR is used as the amplitude of the voltage reference. As discussed in the above, to change the amplihlde of the voltage reference can control the active power flow of the inverter unit; so, the active power regulator can control the output of active power P,+ to follow the active power

reference ehWe effectively. Hence, the operational principle of the proposed load-

sharing scheme can be expressed as follow: (1) Active Power sharing: Using the same active power

reference. All the paralleled units own the same active power

reference p,h,. PR is used to control the output of active power to follow the power reference. Thus, all the units output the same active power.

(2) Reactive Power sharing: Using PLL (phase locked loop) control

Active Power Bus

Fig.7 Control block diagram of ule load-sharing scheme

All the units transfer their Phase angle value to the phase bus respectively. The phase bus extracts the maximum value of the phase angles and transfer back to the inverter units as the input of PLL. Thus, the PLL ensure the voltage phase reference is the same in the paralleled system and when the active power is sharing, the reactive power is

coefficient K,,, , which relies on the parameters of the hardware. Due to the disagreement of the coefficient,

feedback of the unit is disagreement. Hence, the RMS values v, of the load voltage feedback between all units

the load is the Same One, the

are disagreement. It is possible that, the error e, in some

units is positive and e, in the other units is negative when

V, o VIn. This means it is impossible to let all the error

e, equal zero in the same time. Because there exists an integrator Section in the RVR controller, the R V ~ will be in

almost sharing in the paralleled system too. (3) Adaptive control of the load voltage feedback

coefficient In the paralleled system, all the controllen are

implemented by s o h a r e , so the control parameters between parallel inverter modules has no difference. But, in general, it has differences bemeen System panmeters of all the paralleled units, such as the load voltage feedback

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the positive saturation state or in the negative saturation state. Thus, the system cannot run in a right state.

To overcome this problem, an adaptive controller is adopted. The adaptive control law is:

K m = Kshme (eh, - 'n ) + (10)

K , is an adaptive controller. K,,, is the proportional coefficient of the adaptive controller. The product of K , and K,, is used as the actual feedback

coefficient. Obviously, when Chum > P, , the actual

feedback coefficient is less than K , . On the other hand,

when eh, < Pn , the actual feedback coefficient is bigger

than K , . Using the control law will ensure the system to

mn in the right state.

IV. EXPERMENTAL RESULTS

In order to verify the proposed load-sharing scheme, a parallel system with three inverter units has been built. The control unit is implemented based on TMS320F240. The parameters of the unit list as in Table 1.

Table 1: Parameters of inverter unit

Some experimental results are shown in Fig3 to Fig.15, where uo is the load voltage and i, , i2 and i, are the load currents of the three inverter units respectively.

A. Static Performance

Fig.8 and 9 are the static responses of the parallel system with the full resistive load and the full rectifier load respectively. In Fig3 and 9, the load currents i, , i, and i3 are almost the same. Hence, the proposed scheme has good load-sharing performance. Under different load conditions, the THD of load voltage, the RMS precision of load voltage and the precision of load current sharing defined in Equation (1 1) are shown in Fig.10.

AI%=- AI."= x 100% = I L X -1-1 xloo% I , I,,

(11) Where I,, I, and I,o. is the RMS value of the actual

load current of a certain inverter, the expected load current (average load current) and the rated load current,

respectively. It is obvious that the paralleled system has not only

good current-sharing performancm: hut also low THD and high precision of load voltage.

Fig.8 Static response of the parallel syskm wiul full resistive load

a ax a m E% 1m lax l a

Fig.10 Characteristics of the parallel system with resistive load B. Dynumic Pe$ormance

Fig.12 and 13 are the dynamic responses of the parallel

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system when abruptly increasing the load from null to full

Fig.12 Dynamic response of the paralleled system with the resistive load abruptly increasing from null 10 full Fig.14 Dynamic response of the parallel system with the resistive load

abruplly decreasing from full to null

Fig.13 Dynamic response of the parallel system with Le rectifier load abruptly decreasing 6om null to full

responses Of the paralleled system when abruptly decreasing the load from full resistive load and the full rectifier load to null respectively. Fig.12 to Fig.15 show that even if the load changing abruptly, the parallel system still has good current-sharing performance.

Fig.14 and are the Fig.15 Dymamic xspome afthe paallel system with therectifier load abruptly decreasing from full IO null

REFERENCES

[I] Xie LiHua, Su YanMin, “Control of Inverter Parallel Operation”, Power Electronics, Vo1.34, No.4, pp.1-3.

121 David I Perreadt, Roben L Sclden and John G Kassakian, “Frsquency-Based Current-Shanng Techniques for Parallcl Power Converten’, IEEE Transactions on Power Elecmnicr 1998, Vo1.13, N o 4 pp.626-634.

PI Heim Van Der Braeck, Ulrich Boeh, “A Simple Mehod for Parallel Operation oflnverten”, IEEE 1998, pp.144-150.

[4l Yu Meng, ShanXu Dum, Yong Kang and Jian Chen, “Research on Voltage Source Inverten With Wireless Parallel Operation”.

[5l A Tuladhar, H Jin, T Unger and K Much, “Parallel Operation of Single Phase lnvener Modules with No Control Intercome~tims”, IEEE 1997, pp.94-100.

V. CONCLUSIONS

A novel current sharing scheme for paralleled inverter is proposed, The parallel system has characteristics as follows:

(I): Reliable configuration (2): Simple communication (3): Good effect of current sharing (4): Low THD with nonlinear load (5 ) : High precision of load voltage RMS

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